Cathode-follower high-fidelity power amplifier

Abstract

A cathode-follower power output stage is driven by a novel drive stage to provide with low distortion the large signal required to drive the output stage. The drive stage comprises two series connected vacuum tubes which share the large static and dynamic voltages of the drive stage so as to provide a drive signal having a large voltage swing, without generating audible distortion and without subjecting the drive stage tubes to excessively high voltages. The plate of a first drive tube is connected to the cathode of the second drive tube. A load impedance is connected from a B+ power supply terminal to the plate of the second drive tube. The grid of the second drive tube is driven by a signal responsive voltage divider network driven by the plate of the second drive tube. The voltage divider network maintains the grid of the second drive tube at a voltage approximately midway between the voltage of the second drive tube plate and the ground as the plate voltage swings in response to the signal current flowing through the load impedance. The two series connected drive tubes thereby share approximately equally both the quiescent static voltage and the dynamic output voltage of the drive stage as the latter drives the grid of the cathode-follower output stage with the required large amplitude voltage swings.

Claims

1. A high-fidelity vacuum tube power amplifier for driving a loudspeaker to provide music reproduction without audible distortion, said amplifier comprising

at least one drive stage including at least two vacuum tube triodes each having a grid, a plate and a cathode,

said plate of a first of said drive stage triodes being connected in series to the cathode of a second of said drive stage triodes so as to provide a common plate-to-cathode current through both triodes,

a ground,

said first drive stage triode cathode being connected to said ground,

a power supply having at least two supply terminals,

a load impedance connected between a first of said supply terminals and said second drive stage triode plate,

a voltage divider including at least two impedances,

a first of said voltage divider impedances being connected between the plate and grid of said second drive stage triode,

a second of said voltage divider impedances being connected between said second drive stage triode grid and said ground,

means for connecting said drive stage triode grid to a signal source,

a cathode-follower output stage including at least one vacuum-tube power output triode having a grid, a plate and a cathode,

said second drive stage triode plate being drivingly connected to said power output triode grid,

said power output triode plate being connected to a second of said supply terminals,

an output transformer having a primary winding and a secondary winding,

said power output triode cathode being connected to said transformer primary winding,

an output terminal for connection to a loudspeaker, and

said transformer secondary winding being connected to said output terminal,

whereby said drive stage triodes share the total dynamic output voltage swing of the drive stage when amplifying a signal so as to provide with low distortion a large voltage swing required to drive the cathode-follower power output triode.

2. An amplifier as set forth in claim 1 wherein

said output transformer primary winding is connected to said power output triode cathode at one end of said primary winding and the opposite end of said primary winding is connected to said ground whereby said power output triode operates in a single-ended mode, and

means for biasing said power output triode for Class A operation with said power output triode remaining conductive throughout its cycle of voltage swing.

3. An amplifier as set forth in claim 2 wherein

said signal source connecting means comprises a predrive stage including a vacuum tube having a cathode and a plate,

means connecting said predrive stage tube plate to the grid of said first drive stage triode,

a resistor connected between said predrive stage tube cathode and said ground, and

a feedback network having one end connected to said power output triode cathode and another end connected to said predrive stage tube cathode.

4. An amplifier as set forth in claim 3 wherein

said signal source connecting means further comprises an input stage including a vacuum tube having a cathode and a plate,

said input tube plate being connected to the grid of said predrive stage tube,

a resistor connected between said input stage tube cathode and said ground, and

an outer feedback network extending from said output transformer secondary winding to said input stage tube cathode.

5. An amplifier as set forth in claim 1 wherein

said signal source connecting means comprises a predrive stage including a vacuum tube having a cathode and a plate,

said predrive stage tube plate being connected to the grid of said first drive stage triode,

a resistor connected between said predrive stage tube cathode and said ground, and

a feedback network connected to said predrive stage tube cathode for transmitting thereto a feedback signal.

6. An amplifier as set forth in claim 1 and comprising

a second drive stage including at least two vacuum-tube triodes each having a grid, a plate and a cathode,

said plate of a first triode of said second drive stage being connected to the cathode of a second triode of said second drive stage,

said cathode of said first triode of said second drive stage being connected to said ground,

a load impedance connected between said first supply terminal and said plate of said second triode of said second drive stage,

a second voltage divider including at least two impedances,

a first impedance of said second voltage divider being connected between the plate and grid of said second triode of said second drive stage,

a second impedance of said second voltage divider being connected between the grid of said second triode of said second drive stage and said ground,

means for connecting the grid of said first triode of said second drive stage to said signal source to transmit thereto a signal having a phase opposite to that of the signal transmitted to the grid of the first triode of said first-recited drive stage,

said cathode-follower output stage including at least another vacuum-tube power output triode having a grid, a plate and a cathode,

the plate of said second triode of said second drive stage being drivingly connected to the grid of said another power output triode,

the plate of said second power output triode being connected to said second supply terminal,

said output transformer primary winding having opposite ends and a center tap connected to said ground,

each of said power output triode cathodes being connected to a respective end of said transformer primary winding, and

a phase-splitting stage to transmit respective oppositely-phased signals to the grids of said first triodes of said two drive stages,

whereby said amplifier operates in a push-pull mode.

7. A high-fidelity vacuum-tube power amplifier for driving a loudspeaker to provide music reproduction with minimal audible distortion, said amplifier comprising

a signal input terminal,

a power supply having a supply terminal,

at least one drive stage including at least two vacuum tubes connected in series,

a first of said drive tubes having an input electrode,

a second of said drive tubes having an input electrode and an output electrode,

a load impedance connected between said supply terminal and the output electrode of said second drive tube,

circuit means connecting said first drive tube input electrode to said input terminal,

signal responsive means connecting said input and output electrodes of said second drive tube for driving the second drive tube so as to cause the respective varying voltages across the drive tubes to remain approximately equal in response to an input signal to said first drive tube input electrode,

a cathode-follower output stage including at least one power output tube having a grid and a cathode,

said second drive tube output electrode being drivingly connected to said output tube grid,

an output transformer having a primary winding and a secondary winding,

said output tube cathode being connected to said transformer primary winding,

an output terminal for connection to a loudspeaker, and

said transformer secondary winding being connected to said output terminal,

whereby said series-connected drive tubes share the dynamic output voltage swing of the drive stage when amplifying a signal so as to provide a substantially undistorted large voltage swing required to drive the cathode-follower output stage.

8. An amplifier as set forth in claim 7 wherein said signal responsive responsive means comprises

a voltage divider network to maintain the voltage at the input electrode of said second drive stage tube approximately midway between the voltage at the output electrode of said second drive stage tube and the ground voltage.

9. An amplifier as set forth in claim 8 and comprising

a ground,

said output transformer primary winding being connected to said output tube cathode at one end of said winding and the opposite end of said winding being connected to said ground whereby said output tube operates in a single-ended mode, and

means for biasing said output tube for Class A operation with said output tube remaining conductive throughout its cycle of voltage swing.

10. An amplifier as said forth in claim 9 and comprising

a predrive stage including a vacuum tube having a grid, a cathode and a plate,

circuit means connecting said predrive stage tube grid to said input terminal,

said first drive tube having a grid

said predrive stage tube plate being connected to the grid of said first drive tube,

a resistor connected between said predrive stage tube cathode and said ground, and

a feedback network having one end connected to said output tube cathode and its opposite end connected to said predrive stage tube cathode.

11. An amplifier as set forth in claim 7 and comprising

at least one predrive stage including a vacuum tube having a grid, a cathode and a plate,

circuit means connecting said predrive stage tube grid to said input terminal,

said first drive tube having a grid,

said predrive stage tube plate being connected to the grid of said first drive tube,

a resistor connected between said predrive stage tube cathode and said ground, and

a feedback network connected to said predrive stage tube cathode for transmitting a feedback signal thereto.

12. An amplifier as set forth in claim 7 and comprising

a second drive stage including at least two vacuum tubes connected in series,

a first tube of said second drive stage having an input electrode,

a second tube of said second drive stage having an input electrode and an output electrode,

a second load impedance connected between said supply terminal and the output electrode of said second drive tube of said second drive stage,

second signal responsive means connecting said input and output electrodes of said second tube of said second drive stage for driving said second tube in response to a signal to said input electrode of said first tube of said second drive stage,

said cathode-follower output stage including at least a second power output tube having a grid and a cathode,

a ground,

said transformer primary winding having opposite ends and a center tap connected to said ground,

each of said power output tube cathodes being connected to a respective opposite end of said transformer primary winding,

whereby said amplifier operates in a push-pull mode.

13. A vacuum tube amplifier comprising

a drive stage including at least two tubes each having a grid, a cathode and a plate,

said tubes being connected in series with the plate of a first tube connected to the cathode of a second tube so that a common plate-to-cathode current flows through the tubes in series,

means for transmitting a signal to the grid of the first tube,

a ground connected to the cathode of said first tube,

a load impedance connected to the plate of said second tube,

a voltage divider network connecting said plate and grid of said second tube to maintain the respective plate-to-cathode voltages of said tubes approximately equal as the plate voltage of said second tube swings up and down during the amplification of said signal,

a cathode-follower output stage,

and means drivingly connecting the plate of said second tube to said cathode-follower output stage.

14. An amplifier as set forth in claim 13 wherein

said voltage divider network comprises series-connected impedances having values to maintain the voltage at the grid of said second drive stage tube approximately midway between the voltage at the plate of said second drive stage tube and the ground voltage.

15. An amplifier as set forth in claim 14 wherein said voltage divider network comprises

a first impedance connected between said plate and grid of said second tube, and

a second impedance connected between said grid of said second tube and said ground.

16. An amplifier as set forth in claim 15 and comprising

an output transformer having a primary winding connected to said output stage at one end of said winding and the opposite end of said winding being connected to said ground whereby said output tube operates in a single-ended mode, and

means for biasing said output tube for Class A operation with said output tube remaining conductive throughout its cycle of voltage swing.

17. An amplifier as set forth in claim 16 and comprising

a predrive stage including a vacuum tube having a cathode and a plate,

said predrive stage tube plate being connected to the grid of said first drive tube,

a resistor connected between said predrive stage tube cathode and said ground, and

a feedback network extending from said output stage to said predrive stage tube cathode.

18. An amplifier as set forth in claim 17 and comprising

an input terminal,

an input stage including a vacuum tube having a grid, a cathode and a plate,

said input tube grid being connected to said input terminal,

said input tube plate being connected to the grid of said predrive stage tube,

a resistor connected between said input tube cathode and said ground, and

an output terminal for connection to a loudspeaker,

said output transformer having a secondary winding connected to said output terminal, and

an outer feedback network extending from said output terminal to said input tube cathode.

19. An amplifier as set forth in claim 13 and comprising

a predrive stage including a vacuum tube having a cathode and a plate,

said predrive stage tube plate being connected to the grid of said first drive tube,

a resistor connected between said predrive stage tube cathode and said ground, and

a feedback network connected to said predrive stage tube cathode for transmitting thereto a feedback signal.

20. An amplifier has set forth in claim 13 and comprising

a second drive stage including at least two tubes each having a grid, a plate and a cathode,

said tubes of the second drive stage being connected in series with the plate of a first tube connected to the cathode of a second tube so that a common quiescent plate-to-cathode current flows through the tubes in series,

means for transmitting a second signal to the grid of the first tube of the second drive stage,

said second signal being a replication of said first-recited signal but oppositely phased with respect thereto,

the cathode of said first tube of the second drive stage being connected to said ground,

a load impedance connected to the plate of said second tube of the second drive stage,

a second voltage divider network connecting said plate and grid of said second tube of the second drive stage to maintain the respective plate-to-cathode voltages of said tubes approximately equal as the plate voltage of said second tube of the second drive stage swings up and down during amplification of said signal,

said cathode-follower output stage having at least two power output tubes each having a cathode,

an output transformer including a primary winding having each of its opposite ends connected to a respective one of said power output tube cathodes, and

a phase-splitting stage to generate said respective oppositely-phased signals for transmission to the respective grids of said first tubes of said two drive stages,

whereby said amplifier operates in a push-pull mode.